The Plot of the First Two Games: Myst and Riven

In Myst you arrive on the island of Myst and find two brothers,
Sirrus and Achenar, trapped in prison books. They tell you that when their father Atrus found his library destroyed, he imprisoned them both until he could determine which of them committed the crime. Each tries to convince you that the other is guilty. After traveling through four ages, you learn more about the brothers, ultimately discovering that they both collaborated to destroy the library and trap their father in D'ni. At the end of the game, you help Atrus realize the truth and give him access to Myst once again.

Riven starts where Myst ends off. Atrus has edited the Riven book to stabilize the Age long enough to rescue his wife, Catherine,
and evacuate all the other inhabitants. You help him by going to Riven to
find Catherine and trap Gehn, Atrus's father who misuses the power of writing ages.
After traveling through the five islands of Riven, as well as two other ages, you trap Gehn
in a prison book and rescue Catherine, who goes to evacuate Riven's civilians. The only way
to signal Atrus to come to Riven is to open a hole to access the star fissure, a mysterious
opening into the universe. You do this, and Riven begins to literally fall apart. Atrus and
Catherine are reunited and link back to D'ni together. You, on the other hand, fall into the
fissure, your pathway home.